3842
Inorg. Chem. 1985, 24, 3842-3856 Contribution from the Laboratoire de Physique Quantique (CNRS, U A 505) and Laboratoire des Organomttalliques (CNRS, U A 477), UniversitO Paul Sabatier, 3 1062 Toulouse Cedex, France
Binding Phosphinidenes to Transition-Metal Fragments GEORGES T R I N Q U I E R * + and G U Y B E R T R A N D * Received December 21, 1984 Phosphinidenes can be stabilized through internal ?r conjugation as in phosphinophosphinidenes, R2P-P. Such species should be even more efficiently stabilized through transition-metal complexation. This work is a detailed analysis of orbital interactions occurring when H2P-P is bound ( a 1 )to various ML, metallic fragments. The most favorable fragments allow both the donation of the two lone pairs ( u and 7) of the terminal phosphorus and the back-donation of a metal d pair into the empty r*p..p orbital. This is accomplished with the 14-electron species d4 C , ML,, d6 C , ML,, and d8 C3, ML3 and, to a lesser extent, with d6 C, ML,, ds C, ML2 (12 e), and d6 C3, ML4 (14 e). Tetrahedral complexes with a d8 ML3 unit such as Fe(CO), produce the strongest binding. This study is extended to other phosphinidene ligands, (H2N),P-P, H2N-P, H3C-P, and C6HS-P, and to nitrene ligands, HzN-N and H2P-N. Bridging is examined in HzP-P(ML5)2 dinuclear complexes.
I. Introduction Nitrenes and phosphinidenes are known as transient species, and their chemistry has been widely studied.’ These species can be stabilized through complexation on transition-metal fragments. However, a difference appears between nitrenes and phosphinidenes concerning the mode of binding t o the metal centers. Although some terminal nitrene complexes are known,2 t o d a t e t h e only known stable phosphinidene complexes involve bridging (Le. p 2 , pj,or p4) ph~sphinidenes.~Complexes involving terminal phosphinidenes have proven t o be only transient specie^.^-^ A question arises as t o whether t h e r e is a predisposition for phosphinidenes t o behave only as polybridging ligands, inasmuch as the parent valence isoelectronic carbyne ligand (that we consider as R-C:- for electron count purposes) also can give both terminal or bridged complexes.* The present paper presents a theoretical analysis of the bonding occurring in complexes formed by terminal phosphinidenes with various metallic fragments, eventually suggesting t h e most suitable transition-metal f r a g m e n t s t h a t could give such stable terminal complexes. Like nitrenes, phosphinidenes can be stabilized by putting *-donor substituents in the a position. This strategy made possible t h e synthesis of relatively stable a m i n ~ n i t r e n e sand ~ phosphinonitrene.I0 The singlet states are more stabilized t h a n t h e triplet states, so t h a t t h e general propensity t o triplet ground states can be upset if a sufficient *-delocalization possibility is given.” We can illustrate this by giving t h e triplet-singlet splitting in some nitrenes (a negative sign m e a n s a singlet ground state): 36 kcal/mol” 3 1 kcal/mol’ ’
H-N CHO-N C,H,-N H,P-N H,N-N
4 kcal/mol14 -6 kcal/mol” -15 kcal/mol”
In our study we shall consider model phosphinidenes t h a t are already stabilized by internal i~ conjugation. We have chosen to focus on phosphinophosphinidene, H2P-P, which is the isomerical alternative t o diphosphene HP=PH. Diphosphenes have now been observed either as isolated species or in complexed forms on transition m e t a l centers.16J7 The H2P-P ligand is taken in a planar singlet form (which was calculated to lie 29 kcal/mol above
from fragment molecular orbital (FMO) analyses will be largely used. The extended Hiickel parameters and the geometrical (1) (a) Lwowski, W. “Nitrenes”; Wiley-Interscience: New York, 1970. (b) Schmidt, U. Angew. Chem., In?. Ed. Engl. 1975, 14, 523. (2) (a) Lwowsky, W. In “Reactive Intermediates”; Jones, M., Moss, R., Eds.; Wiley: New York, 1978; Chapter 6. (b) Nugent, W. A,; Haymore, B. L. Coord. Chem. Rev. 1980,31, 23. (c) Dehnicke, K.; Strahle, J. Angew. Chem., Int. Ed. Engl. 1981, 20, 413. (d) Mahy, J. P.;
(3)
(4) (5) (6) (7)
(8)
t r a n s - d i p h o ~ p h e n e ) . ” ~We shall see that, a t our level of analysis,
a triplet configuration for the ligand would change t h e bonding very little. Furthermore, i t is presumed t h a t a stable phosphinophosphinidene is likely t o b e a r stabilizing a-donor (or r-acceptor) substituents, t h u s favoring a planar singlet ground state. O t h e r phosphinidene ligands will be considered later: H2N-P, CH3-P, and Ph-P. T h e s t u d y will be extended t o t h e nitrene ligands H,N-N and HIP-N. Finally, we shall consider bridging in t h e simplest case: p 2 dinuclear models. W e base our study on one-electron effects obtained from extended Huckel calculations. Orbital interaction diagrams resulting Laboratoire de Physique Quantique.
* Laboratoire des OrganomCtalliques. 0020-1669/85/1324-3842$01.50/0
(9)
(IO)
Battioni, P.; Mansuy, D.; Fisher, J.; Weiss, R.; Mispelter, J.; Morgenstern-Badarau, I.; Gans, P. J . Am. Chem. SOC.1984, 106, 1699. (e) Maatta, E. A. Inorg. Chem. 1984, 23, 2560. (0Hegedus, L. S.; Kramer, A. Organometallics 1984, 3, 1263. (9) Cotton, F. A,; Duraj, S.A.; Roth, W. J. J . Am. Chem. SOC.1984, 106, 4749. (h) Marchi, A.; Rossi, R.; Duatti, A,; Magon, L.; Casellato, U.; Graziani, R. Transition Met. Chem. (Weinheim, Ger.) 1984, 9, 299. (i) Lin, Y. C.; Knobler, C. B.; Kaesz, H. D. J . Organomet. Chem. 1981, 213, C41. (j) White, R. E.; McCarthy, M. B. J . A m . Chem. SOC.1984, 106, 4922. (k) Hsieh, T. C.; Gebreyes, K.; Zubieta, J. J . Chem. SOC.,Chem. Commun. 1984, 1172. (I) Brennan, J. G.; Andersen, R. A. J . Am. Chem. Soc. 1985,107,514. (m) Chan, D. M. T.; Fultz, W. C.; Nugent, W. A.; Roe, D. C.; Tulip, T. H. J . Am. Chem. SOC.1985, 107, 251. (n) Gross, M. E.; Johnson, C. E.; Maroney, M. J.; Trogler, W. C. Inorg. Chem. 1984, 23, 2968. (0)Chatt, J.; Dosser, R. J.; King, F.; Leigh, G. J. J. Chem. SOC.,Dalton Trans. 1976,2435. (p) For an MO treatment of transition metal complexes with ligands N, NN, or NNR, see: Dubois, D. L.; Hoffmann, R. Nouv. J . Chim. 1977, I , 479. (a) For a review, see: Marinetti, A. Thesis, UniversitC Pierre-et-Marie-Curie, Paris, 1984. (b) Duttera, M. R.; Day, V. W.; Marks, T. J. J . Am. Chem. Soc. 1984,106,2907. (c) Kwek, K.; Taylor, N. J.; Carty, A. J. J . Am. Chem. SOC.1984, 106, 4636. (d) Seyferth, D.; Withers, H. P., Jr. Organometallics 1982, I , 1294. (e) Williams, G. D.; Geoffroy, G. L.; Whittle, R. R. J . Am. Chem. SOC.1985, 107, 729. (0For closo 6-vertex clusters containing two trans p4-PhPunits, see: Halet, J. F.; Hoffmann, R.; Saillard, J. Y . Inorg. Chem. 1985, 24, 1695. Marinetti, A.; Mathey, F.; Fischer, J.; Mitschler, A. J . Chem. Sot. Chem. Commun. 1982,667. Marinetti, A.; Mathey, F.; Fischer, J.; Mitschler, A. J . Am. Chem. Sac. 1982, 104, 4484. (a) Marinetti, A,; Mathey, F. Organometallics 1984, 3, 456. (b) Marinetti, A.; Mathey, F. Organometallics 1984, 3, 1492. Nakazawa, H.; Buhro, W. E.; Bertrand, G.; Gladysz, J. A. Inorg. Chem. 1984, 23, 3431. (a) Purcell, K. F.; Kotz, J. C. “Inorganic Chemistry”; W. B. Saunders: Philadelphia, 1977; p 863. (b) Collman, J. P.; Hegedus, L. S. “Principles and Applications of Organotransition Metal Chemistry”; University Science Books: Mill Valley, CA 1980; p 103. (c) Clauss, A. D.; Shapley, J. R.; Wilker, C. N.; Hoffmann, R. Organometallics 1984, 3, 619. (d) Ros, J.; Solans, X.;Font-Altaba, M.; Mathieu, R. Organometallics 1984, 3, 1014. (e) Carriedo, G. A,; Howard, J. A. K.; Stone, F. G. A. J . Chem. SOC.,Dalton Trans. 1984,8, 1555. Jeffery, J. C.; Laurie, J. C. V.; Moore, I.; Razay, H.; Stone, F. G. A. Ibid. 1984, 8, 1563. Jeffery, J. C.; Moore, I.; Stone, F. G. A. Ibid. 1984, 8, 1571. Jeffery, J. C.; Moore, I.; Razay, H.; Stone, F. G. A. Ibid. 1984,8, 158 1. Carriedo, G. A,; Howard, J. A. K.; Marsden, K.; Stone, F. G. A,; Woodward, P. Ibid. 1984, 8, 1589. Carriedo, G. A,; Jeffery, J. C.; Stone, F. G . A. Ibid. 1984, 8, 1597. Behrens, U.; Stone, F. G. A. Ibid. 1984, 8, 1605. See, for instance: Sylwester, A. P.; Dervan, P. B. J . Am. Chem. SOC. 1984, 106, 4648 and reference 4 in this work. (a) Sicard, G.; Baceiredo, A.; Bertrand, G.; Majoral, J. P. Angew. Chem., In?.Ed. Engl. 1984, 23, 459. (b) Baceiredo, A,; Bertrand, G.; Majoral, J. P.; El Anba, F.; Manuel, G. J . Am. Chem. Sac. 1985, 107,
3945. (11) Trinquier, G. J . Am. Chem. SOC.1982, 104, 6969. (1 2) H u k r , K. P.; Herzberg, G. ‘Constants for Diatomic Molecules”; Van Nostrand-Reinhold: New York, 1979. (13) Mavridis, A,; Harrison, J. F. J . Am. Chem. Sac. 1980, 102, 7651.
0 1985 American Chemical Society
Inorganic Chemistry, Vol. 24, No. 23, I985 3843
Phosphinidene Binding
H\ /
I
P
-10
n' -11
2b.
-
T
-12
-13
>
0
w
- 14 -15
-16
-17
Figure 1. Relevant molecular orbitals of the phosphinophosphinidene ligand.
assumptions used in this work are given in the Appendix. Before considering in detail the bonding abilities of different classes of metallic fragments toward H2P-P-starting with those derived from the octahedron in which one, two, and then three ligands are successively removed-we shall examine the electronic structure of our model ligand. The four relevant orbitals that can be involved in the bonding to the metal are plotted in Figure 1 . The lower MO of this set, 1bl, corresponds to the phosphino r lone pair, conjugated with a p orbital of the neighbor phosphorus atom (1). In an extended Huckel calculation, both phosphorus n
Figure 2. Schematized interaction diagram for d6 H2PP-ML5. p atomic orbital on the terminal phosphorus. Since Ibl is not located mainly on the terminal phosphorus, this orbital will not be involved in the q1 bonding but will be involved in v2 bonding. Next, we have two orbitals corresponding to the phosphinidene lone pairs: a l is a sp-hybridized u lone pair pointing away from the P-P bond as schematized in 2. b2, the HOMO, is a p lone pair (3). Figure 1 shows that b2 has a component on the
1
atoms bear the same coefficients as occurs also for the antibonding counterpart 2bl of this r system (see Figure 1). Actually, as illustrated by a b initio calculations, there is a dissymetry and 1 bl is more the phosphino lone pair whereas 2bl is more the vacant P. S . ; Brauman, J. I. J. Am. Chem. SOC.1984, 106, 3443. Drzaic, P. S.; Brauman, J. I. J . Phys. Chem. 1984, 88, 5285. (15) (a) Davis, J. H.; Gcddard, W. A,, 111. J. Am. Chem. SOC.1977, 99, 7111. (b) For a review on singlet-triplet separations in nitrenes, see: Davidson, E. R.In "Diradicals"; Borden, W. T., Ed.; Wiley: New York, 1982; Chapter 2, p 73. (a) Cowley, A. H. Polyhedron 1984, 3, 389. (b) Cowley, A. H.; Kilduff, J. A,; Lasch, J. G.; Mehrotra, S . K.; Norman, N. C.; Pakulski, M.; Whittlesey, B. R.;Atwood, J. L.; Hunter, W. E. Znorg. Chem. 1984, 23, 2582. (c) Cowley, A. H.; Kilduff, J. E.; Lasch, J. G.; Pakulski, M.; Ando, F.; Wright, T. C. Organometallics 1984, 3, 1044. (d) See also, for a review: Ranaivoniatovo, H. J. Thke de troisisme cycle, Univenitd Paul Sabatier, Toulouse, France, 1984. (17) For theoretical studies on diphosphene HP=PH, see: (a) Yoshifuji, M.; Shibayama, K.; Inamoto, N. J . Am. Chem. SOC.1983,105,2495. (b) Lee, J. G.; Cowley, A. H.; Boggs, J. E. Inorg. Chim. Acta 1983, 77, L61. (c) Galasso, V. Chem. Phys. 1984,83,407. (d) Ha, T. K.; Nguyen, M. T.; Ruelle, P. Chem. Phys. 1984, 87, 23. (14) Drzaic,
2
3
phosphino group. In fact, this orbital should also have a significant coefficient on a d orbital of the adjacent phosphorus, as has been reported elsewhere for H2P-N." This interaction is not taken into account at our level since we do not include d orbitals in the basis sets for main-group atoms. The LUMO, 2bl, is the remaining vacant p atomic orbital of the terminal phosphorus (4). It is the antibonding counterpart of Ibl. This orbital may receive electrons from a metal, thus contributing to back-bonding.
4
It can be pointed out in Figure 1 that if b2 is empty, the orbital pattern of singlet vinylidene R2C=C: is obtained. Therefore, from simple electron-count considerations, the various known dm ML,-CCR2 systemsI8 (e.g. with a d6 ML5 fragment) suggest the
3844 Inorganic Chemistry, Vol. 24, No. 23, 1985
Trinquier and Bertrand
Table I. Summary of the Results of Extended Huckel Calculations for Complexes of H,P-P with Nonfavorable Fragments tot. overlap
0.81
gross charge transfer, e A(HOMOP-P P - M P - M LUMO), eV 0.56 1.06 1.06 0.03 0.90 1.16
0.88 0.79 0.28
1.03 1.12 1.09
POP.
AE,eV P-M
fragments
WHS5-
2.88 1.21 3.34 1.27 0.63
W(C0)5 WH,CISW(CO),CIPtCI
-
-13
m
u1
-14
-15
-16
a
-17
Figure 6. Interaction diagram for H2PP-Fe(C0)3. CI
CI
22
23
corresponding orbital interaction diagram is given in Figure 9. The two orbitals of H2P-P that are involved in bonding interactions are now occupied 1b, and empty 2bl T * ~ The ~ . symmetry of the whole complex is only C,,allowing orbital mixings, but the main interactions are clearly visible in Figure 10. The main bonding interaction is by far that which involves the empty orbital, which overlaps well with the occupied orbital at the bottom of the "tZg"set of PtC13-. The interaction involving occupied rPp is less energy gaining, partly due to a secondary interaction with a lower occupied orbital of PtC13-. The a, lone pair of H2P-P still has the right symmetry to delocalize into the empty a, orbital of PtClF, giving a significant stabilizing interaction. The b2 lone pair of HIP-P encounters only occupied orbitals of the same symmetry (a") in PtCl;. This will give a destabilizing interaction. Finally, the binding energy is very weak, and both Pt-P overlap populations are weak (see Table I). As expected from the predominant ~ * - dmixing, the P-P bond is much weakened and the Pt direction. resulting electron transfer is in the P Note that in Zeise's salt the A and ?r* levels of ethylene are both higher in energy (-13.3 and -8.0 eV, respectively), allowing a stronger rcc-al bonding interaction while the r*cc-bl bonding interaction should be weaker. The last point is generally compensated by a cis bending of the olefin substituents away from the metal. This results in a lowering of the A* level and an increase
-
(28) Actually, the P-P overlap population in both H2P-P-NiF3* and H2PP-NiF4 is calculated at 1.35, which is above the value for isolated HIP-P (1.34). This is due to the fact that in these complexes we have not populated the b, MO of H2P-P, which is P-P antibonding, while we have depopulated the b2 MO, which is slightly P-P antibonding. (29) For MO treatments of transition-metal-complexed olefins, see: (a) Albright, T. A.; Hoffmann, R.; Thibeault, J. C.; Thorn, D. L. J . Am. Chem. SOC.1979,101, 3801. (b) Eisenstein, 0.; Hoffmann, R. J . Am. Chem. SOC.1981,103,4308. (c) Hoffmann, R.Science (Washingfon, D.C.) 1981,211, 995.
of its overlap with the metal b, orbital.29a Such a puckering of the hydrogens in our case (Le. a pyramidalization of PH2) may also be expected to strengthen the bonding by improving the b,-b, interaction. The u lone pair on the terminal phosphorus is not closely involved in the bonding and remains available for coordinating another metal, giving a dinuclear complex (24). Since this orbital
I ML"
24
is low in energy (-1 5.8 eV; see Figure 9), the M'L', group should have a low a,-type acceptor orbital. This might be realized for instance with AuR+.~O If we tilt H2P-P to obtain the eclipsed form of the 7' complex (25), the strength of the binding is not dramatically changed (see Table I); 0.3 eV has been gained in energy. The two.bonding interactions are now al-al and b,-b,; an orbital interaction diagram shows that the b2-b2 interaction is still very repulsive through a three-orbital six-electron interaction. The staggered form of the q1 complex (26)turns out to be more stable. The orbital interaction diagram shows a less strong bl-b, bonding interaction. There is, at the same time, a less repuslive b2-b2 interaction (involving only two orbitals and four electrons), the a,-al interaction remaining the same. This results in stronger binding, as can be seen in Table I. In summary, with PtC13-, R2P-P forms a q1 staggered complex, rather than a q2 complex. This 9' complex however is much more stable when the PtCI3 fragment has a d6 electron count (Le. PC13+) as was discussed previously. ~~
(30) Komiya, S.; Albright, T. A,; Hoffmann, R.; Kochi, J. K. J . Am. Chem. SOC.1976,98, 1255.
Inorganic Chemistry, Vol. 24, No. 23, 1985 3851
Phosphinidene Binding
P
\ / 'P'
I
P
P
dl
CI
25
26
IX. (H2N)2P-P Ligand In the same way that a a conjugation stabilizes H2P-P with respect to H-P, any further substitution of hydrogen by a a-donor group, such as an amino group, should intrinsically stabilize the >P-P framework or any valence isoelectronic analogues such as >N-P, >P-N, or >N-N. Does this enhanced conjugation strengthen or weaken the bonding of R2P-P to a metal fragment? Figure 10 displays what is changed in the M O pattern of H2P-P when the hydrogens are replaced by amino groups to give planar (H2N),P-P. The occupied a, and b2 orbitals, which correspond to the two lone pairs of the phosphinidene end, have not been dramatically modified either in energy or in shape. The a orbitals now form a trimethylenemethane-like set. l b l , the in-phase combination of p atomic orbitals, has been pushed down in energy, but this is without important consequences for the bonding. The empty a * orbital, 3bl, which is important for the bonding, has been pushed up in energy by more than 2 eV31 while its nature is inevitably less localized on the terminal phosphorus. Both effects are enough to reduce the bl-bl bonding interaction in a complex, through poor energy matching and smaller bl-bl overlap. But there is more. In the a set of (H2N),P-P, there are two high-lying occupied orbitals. a, is mainly localized on the N2P- end and should not interfere in the bonding. On the other hand 2bl has a large coefficient on the terminal phosphorus and should weaken the bonding through mixing in the bl-bl bonding interaction or through repulsive interaction with an eventual occupied bl orbital of the metallic fragment. The orbital interaction diagram for (H2N)2PP-W(C0)4 is shown in Figure 1 1. Let us compare it with the same diagram for H2PP-W(C0)4 (Figure 4). The al-al bonding interaction is less stabilizing. This may be ascribed to a weaker (allal) overlap. As expected from the identical bz orbitals in H2P-P and (H,N),P-P, the bz-b2 bonding interactions are nearly identical in the two complexes. Much is changed in the bl-bl interaction. With H2P-P (Figure 4) we had a clear-cut two-orbital interaction whereas with (H,N),P-P (Figure 11) we have a four-orbital six-electron interaction that does not bring any energy benefit. The calculated indexes listed in Table V all indicate less strong binding of (H2N),P-P. The binding energy has dropped to 2.9 eV (vs. 5.1 eV for H2P-P); the P-W total overlap population has dropped to 1.1 (vs. 1.2 for H2P-P). As expected from the poor involvement of the empty bl orbital in the bonding, the resulting P W electron transfer is enhanced (0.52 vs. 0.17) and the P-P overlap population is not reduced with respect to its value in the isolated molecule (1.20). A similar analysis can be performed for the complex (H2N)2P-P-Fe(CO)3. The calculated values given in Table V also indicate that the strength of the P-Fe link is largely reduced in the diamino complex. Note that this time, the decrease of empty bl orbital involvement in the bonding helps to reverse the direction of the whole electron transfer. For this complex we give in Table VI the values of the overlap for the three couples of relevant orbitals. This illustrates what we said above on the nature of some orbitals in (H,N),P-P. The last comment we shall make on all these numerical values concerns the HOMO-LUMO gaps. In the complexes with
-
W(CO)4, the HOMO-LUMO gap is virtually the same if the ligand is H2P-P or (H2N),P-P. Figures 4 and 11 are self-explanatory for this. On the other hand, Figure 6 makes clear why the HOMO-LUMO gap in the complexes with Fe(C0)3 varies with the amount of the bl-bl bonding interaction. Actually, this gap is reduced by half if the ligand is (H2N)*P-P instead of HzP-P. This is due to a poor b,-e interaction, inducing a higher a" H O M O level. By analogy with trimethylenemethane, a a complexation (Le. q4 binding) can be considered for our ligand. Trimethylenemethane has four a electrons and binds typical d8 ML, fragments such as Fe(C0)3.32 (H2N),PP has six a electrons and therefore may be expected to bind d6 ML3 fragments (27). Three examples
/yL
L
L
27
have been studied here: W(CO),, Fe(C0)32+,and Ni(C0):'. The results are reported in Table V. The calculated binding energies are weak in any case, suggesting that this mode of binding is not favorable for our ligand. The interaction diagrams show that the bonding interactions (bl-al and b,,a,-e) remain weak even when the energy match is favorable such as with Ni(C0):'. The main reason for the weak interactions is the poor overlap between the relevant orbitals. Although bringing the planar ligand closer to the metal (for metal to central phosphorus distances shorter than 2 A) does not improve the bonding (the binding energy is reduced), a puckering of the ligand, as observed in the trimethylenemethane complexes, should greatly strengthen the bonding. We have just shown that the binding of a phosphinophosphinidene to a transition-metal fragment is less strong when the phosphino part bears a-donor substituents such as amino groups. At this point an important remark must be made. In searching for potentially viable R2PP-MLn complexes, one must keep in mind that the R2P-P part must possess a certain intrinsic stability, otherwise the stabilization brought by the metal complexation may not be sufficient to trap it in its phosphinophosphinidene form. Since this essential requirement can be met by using a-donor substituents, it is reasonable to think that (R,N),P-P ligands should form metallic complexes that are more viable than the ones formed from simple R2P-P ligands.
X. H2N-P, H,N-N, and H2P-N Ligands Let us consider now some singlet species that are valence isoelectronic with H2P-P and in which one or two phosphorus atoms are substituted by nitrogen. These are H,N-P, H,N-N, and H2P-N. The relevant energy levels for these species are shown in Figure 12. Some pertinent overlaps with the Fe(CO)3 fragment are listed in Table VI, and the numerical results for the corresponding complexes are reported in Table V. Aminophosphinidene. The (bile) and ( a l l a l ) overlaps are slightly smaller than in phosphinophosphinidene. On the other hand, the empty bl level is significantly lower in energy, which should give, in the complex with Fe(CO),, (1) an increased binding energy and (2) an increased P Fe electron migration through bl-bl mixing. This appears in the calculated values listed in Table V. To sum up, we can say, however, that both phosphino- and aminophosphinidene exhibit comparable properties for bonding to Fe(CO),. Aminonitrene. Table VI shows that all overlaps are much weaker. The energy levels of H,N-N are close to those of H,P-P, except the HOMO, b, orbital, which is higher and should afford
-
~
(31) The resulting increase of the HOMO-LUMO gap is, in our approach, what favors a closed-shell singlet ground state in (R2N)*P-P.
(32) Albright, T. A,; Hofmann, P.; Hoffmann, R. J . Am. Chem. SOC.1977, 99,7546.
3852 Inorganic Chemistry, Vol. 24, No. 23, 1985
Trinquier and Bertrand
/h\
a”
A
a’
a’
Figure 7. Interaction diagrams for H2PP-FeCp- (right) and H2PP-CoCp (left)
1 P
P
-11
%
61
CI
-10
-
-11
-
-12
-
-1 3
-
-14
-
a
n
I
I
gt
-10
-
H-P=P H--.
I
I
-12
H- >P=P H-
-13
UI
- 14
->.u UJ
-15
- 16 -17
Figure 8. Interaction diagram for H2PP-NiF4.
a better energy match. Poor overlaps prevail in making all bonding interactions weaker. The complex H,NN-Fe(CO), is therefore bound much less than the complex H,PP-Fe(CO), as indicated
Figure 9. Interaction diagram for q2-H2PP-PtCI
